Decoded picture buffer indexing

ABSTRACT

A video decoder is configured to remove pictures from a decoded picture buffer based on the value of an explicitly coded syntax element. A video decoder may be configured to decode a syntax element indicating a picture to remove from a decoded picture buffer, and remove the first picture from the DPB. The video decoder may then decode a current picture, and store the decoded current picture in the DPB.

This application claims the benefit of U.S. Provisional Application No.62/863,375, filed Jun. 19, 2019, the entire content of which isincorporated by reference herein.

TECHNICAL FIELD

This disclosure relates to video encoding and video decoding.

BACKGROUND

Digital video capabilities can be incorporated into a wide range ofdevices, including digital televisions, digital direct broadcastsystems, wireless broadcast systems, personal digital assistants (PDAs),laptop or desktop computers, tablet computers, e-book readers, digitalcameras, digital recording devices, digital media players, video gamingdevices, video game consoles, cellular or satellite radio telephones,so-called “smart phones,” video teleconferencing devices, videostreaming devices, and the like. Digital video devices implement videocoding techniques, such as those described in the standards defined byMPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced VideoCoding (AVC), ITU-T H.265/High Efficiency Video Coding (HEVC), andextensions of such standards. The video devices may transmit, receive,encode, decode, and/or store digital video information more efficientlyby implementing such video coding techniques.

Video coding techniques include spatial (intra-picture) predictionand/or temporal (inter-picture) prediction to reduce or removeredundancy inherent in video sequences. For block-based video coding, avideo slice (e.g., a video picture or a portion of a video picture) maybe partitioned into video blocks, which may also be referred to ascoding tree units (CTUs), coding units (CUs) and/or coding nodes. Videoblocks in an intra-coded (I) slice of a picture are encoded usingspatial prediction with respect to reference samples in neighboringblocks in the same picture. Video blocks in an inter-coded (P or B)slice of a picture may use spatial prediction with respect to referencesamples in neighboring blocks in the same picture or temporal predictionwith respect to reference samples in other reference pictures. Picturesmay be referred to as frames, and reference pictures may be referred toas reference frames.

SUMMARY

Video coders (e.g., video encoders and decoders) may use a decodedpicture buffer (DPB) to store decoded pictures. The decoded pictures maybe output for display. In addition, the decoded pictures stored in theDPB may be used as reference pictures for inter-prediction. Pictures arestored in the DPB as long as the pictures can still be used as referencepictures. However, pictures are eventually removed from the DPB to makeroom for currently decoded pictures. In general, pictures are removedfrom a DPB in a picture marking process based on the pictures indicatedin one or more reference picture lists. That is, if a picture is not ina reference picture list and is not marked as needed for output, thevideo coder may mark such a picture as “unused for reference” and removesuch a picture from the DPB.

In general, this disclosure describes techniques for DPB management,including techniques for removing pictures from a DPB. In some examples,this disclosure describes DPB indexing techniques. Rather thanperforming a picture marking process where pictures are marked as unusedfor reference and subsequently removed from the DPB based on pictures ina reference picture list, a video coder may code a DPB index thatexplicitly indicates which picture(s) may be removed from the DPB tomake room for storing the currently coded picture.

The DPB indexing techniques of the disclosure may provide moreflexibility in the signaling of reference picture lists, since there isno need to indicate what pictures to keep. Signaling which pictures tokeep in the DPB by including such pictures in a reference picture listhas associated overhead, since some pictures are just signaled in thereference picture list of a current picture to be kept for thesubsequent pictures (e.g., carried over), even if such pictures are notused for inter prediction of the current picture. Rather, in thetechniques of this disclosure, a video encoder may explicitly signal anindex (e.g., a DPB index) to a picture that can be replaced or bumped(e.g., overwritten or otherwise discarded) from the DPB. In this way,only the pictures actually used for reference need to be signaled in thereference picture list. As such, overhead signaling is reduced andcoding efficiency is increased.

Furthermore, the process for removing pictures from a DPB is simplified,as the picture or pictures to be removed are explicitly signaled with aDPB index. As such, no picture marking process based on referencepicture list is needed. Furthermore, pictures would also not need to bechecked to determine if they are still needed for output, since a videoencoder would not signal a DPB index of a picture that is to be removedfrom the DPB that is still needed for output.

In one example, a method includes decoding a first syntax elementindicating a first picture to remove from a DPB, removing the firstpicture from the DPB, decoding a current picture, and storing thedecoded current picture in the DPB.

In another example, a device includes a memory configured to store videodata, and one or more processors implemented in circuitry and incommunication with the memory, the one or more processors configured todecode a first syntax element indicating a first picture to remove froma DPB, remove the first picture from the DPB, decode a current picture,and store the decoded current picture in the DPB.

In another example, a device includes means for decoding a first syntaxelement indicating a first picture to remove from a DPB, means forremoving the first picture from the DPB, means for decoding a currentpicture, and means for storing the decoded current picture in the DPB.

In another example, a computer-readable storage medium is encoded withinstructions that, when executed, cause a programmable processor todecode a first syntax element indicating a first picture to remove froma DPB, remove the first picture from the DPB, decode a current picture,and store the decoded current picture in the DPB.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system that may perform the techniques of this disclosure.

FIGS. 2A and 2B are conceptual diagrams illustrating an example quadtreebinary tree (QTBT) structure, and a corresponding coding tree unit(CTU).

FIG. 3 is a block diagram illustrating an example video encoder that mayperform the techniques of this disclosure.

FIG. 4 is a block diagram illustrating an example video decoder that mayperform the techniques of this disclosure.

FIG. 5 is a flowchart illustrating an example encoding method of thedisclosure.

FIG. 6 is a flowchart illustrating an example decoding method of thedisclosure.

FIG. 7 is a flowchart illustrating an example decoded picture buffer(DPB) management method performed at a video encoder according to thetechniques of the disclosure.

FIG. 8 is a flowchart illustrating an example decoded picture buffermanagement (DPB) method performed at a video decoder according to thetechniques of the disclosure.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for decoded picturebuffer (DPB) management, including techniques for removing pictures froma DPB. In some examples, this disclosure describes DPB indexingtechniques. Rather than performing a picture marking process wherepictures are marked as unused for reference and subsequently removedfrom a DPB based on pictures in a reference picture list, a video codermay code a DPB index that explicitly indicates which picture may beremoved from the DPB to make room for storing the currently codedpicture.

The DPB indexing techniques of the disclosure may provide moreflexibility in the signaling of reference picture lists, since there isno need to indicate what pictures to keep. Signaling which pictures tokeep in the DPB by including such pictures in a reference picture listhas associated overhead, since some pictures are just signaled in thereference picture list of a current picture to be kept for thesubsequent pictures (e.g., carried over), even if such pictures are notused for inter prediction of the current picture. Rather, in thetechniques of this disclosure, a video encoder may explicitly signal anindex (e.g., a DPB index) to a picture that can be replaced or bumped(e.g., overwritten or otherwise discarded) from the DPB. In this way,only the pictures actually used for reference need to be signaled in thereference picture list. As such, overhead signaling is reduced andcoding efficiency is increased.

Furthermore, the process for removing pictures from a DPB is simplified,as the picture or pictures to be removed are explicitly signaled, e.g.,by an encoder for a decoder, with a DPB index. As such, no picturemarking process based on reference picture list is needed. Furthermore,pictures would also not need to be checked to determine if they arestill needed for output, since a video encoder would not signal a DPBindex of a picture that is to be removed from the DPB if that picture isstill needed for output.

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system 100 that may perform the techniques of this disclosure.The techniques of this disclosure are generally directed to coding(encoding and/or decoding) video data. In general, video data includesany data for processing a video. Thus, video data may include raw,unencoded video, encoded video, decoded (e.g., reconstructed) video, andvideo metadata, such as signaling data.

As shown in FIG. 1, system 100 includes a source device 102 thatprovides encoded video data to be decoded and displayed by a destinationdevice 116, in this example. In particular, source device 102 providesthe video data to destination device 116 via a computer-readable medium110. Source device 102 and destination device 116 may comprise any of awide range of devices, including desktop computers, notebook (i.e.,laptop) computers, tablet computers, set-top boxes, telephone handsetssuch smartphones, televisions, cameras, display devices, digital mediaplayers, video gaming consoles, video streaming device, or the like. Insome cases, source device 102 and destination device 116 may be equippedfor wireless communication, and thus may be referred to as wirelesscommunication devices.

In the example of FIG. 1, source device 102 includes video source 104,memory 106, video encoder 200, and output interface 108. Destinationdevice 116 includes input interface 122, video decoder 300, memory 120,and display device 118. In accordance with this disclosure, videoencoder 200 of source device 102 and video decoder 300 of destinationdevice 116 may be configured to apply the techniques for decoded picturebuffer indexing. Thus, source device 102 represents an example of avideo encoding device, while destination device 116 represents anexample of a video decoding device. In other examples, a source deviceand a destination device may include other components or arrangements.For example, source device 102 may receive video data from an externalvideo source, such as an external camera. Likewise, destination device116 may interface with an external display device, rather than includingan integrated display device.

System 100 as shown in FIG. 1 is merely one example. In general, anydigital video encoding and/or decoding device may perform techniques fordecoded picture buffer indexing. Source device 102 and destinationdevice 116 are merely examples of such coding devices in which sourcedevice 102 generates coded video data for transmission to destinationdevice 116. This disclosure refers to a “coding” device as a device thatperforms coding (encoding and/or decoding) of data. Thus, video encoder200 and video decoder 300 represent examples of coding devices, inparticular, a video encoder and a video decoder, respectively. In someexamples, devices 102, 116 may operate in a substantially symmetricalmanner such that each of devices 102, 116 include video encoding anddecoding components. Hence, system 100 may support one-way or two-wayvideo transmission between video devices 102, 116, e.g., for videostreaming, video playback, video broadcasting, or video telephony.

In general, video source 104 represents a source of video data (i.e.,raw, unencoded video data) and provides a sequential series of pictures(also referred to as “frames”) of the video data to video encoder 200,which encodes data for the pictures. Video source 104 of source device102 may include a video capture device, such as a video camera, a videoarchive containing previously captured raw video, and/or a video feedinterface to receive video from a video content provider. As a furtheralternative, video source 104 may generate computer graphics-based dataas the source video, or a combination of live video, archived video, andcomputer-generated video. In each case, video encoder 200 encodes thecaptured, pre-captured, or computer-generated video data. Video encoder200 may rearrange the pictures from the received order (sometimesreferred to as “display order”) into a coding order for coding. Videoencoder 200 may generate a bitstream including encoded video data.Source device 102 may then output the encoded video data via outputinterface 108 onto computer-readable medium 110 for reception and/orretrieval by, e.g., input interface 122 of destination device 116.

Memory 106 of source device 102 and memory 120 of destination device 116represent general purpose memories. In some examples, memories 106, 120may store raw video data, e.g., raw video from video source 104 and raw,decoded video data from video decoder 300. Additionally oralternatively, memories 106, 120 may store software instructionsexecutable by, e.g., video encoder 200 and video decoder 300,respectively. Although shown separately from video encoder 200 and videodecoder 300 in this example, it should be understood that video encoder200 and video decoder 300 may also include internal memories forfunctionally similar or equivalent purposes. Furthermore, memories 106,120 may store encoded video data, e.g., output from video encoder 200and input to video decoder 300. In some examples, portions of memories106, 120 may be allocated as one or more video buffers, e.g., to storeraw, decoded, and/or encoded video data.

Computer-readable medium 110 may represent any type of medium or devicecapable of transporting the encoded video data from source device 102 todestination device 116. In one example, computer-readable medium 110represents a communication medium to enable source device 102 totransmit encoded video data directly to destination device 116 inreal-time, e.g., via a radio frequency network or computer-basednetwork. Output interface 108 may modulate a transmission signalincluding the encoded video data, and input interface 122 may demodulatethe received transmission signal, according to a communication standard,such as a wireless communication protocol. The communication medium maycomprise any wireless or wired communication medium, such as a radiofrequency (RF) spectrum or one or more physical transmission lines. Thecommunication medium may form part of a packet-based network, such as alocal area network, a wide-area network, or a global network such as theInternet. The communication medium may include routers, switches, basestations, or any other equipment that may be useful to facilitatecommunication from source device 102 to destination device 116.

In some examples, source device 102 may output encoded data from outputinterface 108 to storage device 112. Similarly, destination device 116may access encoded data from storage device 112 via input interface 122.Storage device 112 may include any of a variety of distributed orlocally accessed data storage media such as a hard drive, Blu-ray discs,DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or anyother suitable digital storage media for storing encoded video data.

In some examples, source device 102 may output encoded video data tofile server 114 or another intermediate storage device that may storethe encoded video generated by source device 102. Destination device 116may access stored video data from file server 114 via streaming ordownload. File server 114 may be any type of server device capable ofstoring encoded video data and transmitting that encoded video data tothe destination device 116. File server 114 may represent a web server(e.g., for a website), a File Transfer Protocol (FTP) server, a contentdelivery network device, or a network attached storage (NAS) device.Destination device 116 may access encoded video data from file server114 through any standard data connection, including an Internetconnection. This may include a wireless channel (e.g., a Wi-Ficonnection), a wired connection (e.g., digital subscriber line (DSL),cable modem, etc.), or a combination of both that is suitable foraccessing encoded video data stored on file server 114. File server 114and input interface 122 may be configured to operate according to astreaming transmission protocol, a download transmission protocol, or acombination thereof.

Output interface 108 and input interface 122 may represent wirelesstransmitters/receivers, modems, wired networking components (e.g.,Ethernet cards), wireless communication components that operateaccording to any of a variety of IEEE 802.11 standards, or otherphysical components. In examples where output interface 108 and inputinterface 122 comprise wireless components, output interface 108 andinput interface 122 may be configured to transfer data, such as encodedvideo data, according to a cellular communication standard, such as 4G,4G-LTE (Long-Term Evolution), LTE Advanced, 5G, or the like. In someexamples where output interface 108 comprises a wireless transmitter,output interface 108 and input interface 122 may be configured totransfer data, such as encoded video data, according to other wirelessstandards, such as an IEEE 802.11 specification, an IEEE 802.15specification (e.g., ZigBee™), a Bluetooth™ standard, or the like. Insome examples, source device 102 and/or destination device 116 mayinclude respective system-on-a-chip (SoC) devices. For example, sourcedevice 102 may include an SoC device to perform the functionalityattributed to video encoder 200 and/or output interface 108, anddestination device 116 may include an SoC device to perform thefunctionality attributed to video decoder 300 and/or input interface122.

The techniques of this disclosure may be applied to video coding insupport of any of a variety of multimedia applications, such asover-the-air television broadcasts, cable television transmissions,satellite television transmissions, Internet streaming videotransmissions, such as dynamic adaptive streaming over HTTP (DASH),digital video that is encoded onto a data storage medium, decoding ofdigital video stored on a data storage medium, or other applications.

Input interface 122 of destination device 116 receives an encoded videobitstream from computer-readable medium 110 (e.g., storage device 112,file server 114, or the like). The encoded video bitstream may includesignaling information defined by video encoder 200, which is also usedby video decoder 300, such as syntax elements having values thatdescribe characteristics and/or processing of video blocks or othercoded units (e.g., slices, pictures, groups of pictures, sequences, orthe like). Display device 118 displays decoded pictures of the decodedvideo data to a user. Display device 118 may represent any of a varietyof display devices such as a cathode ray tube (CRT), a liquid crystaldisplay (LCD), a plasma display, an organic light emitting diode (OLED)display, or another type of display device.

Although not shown in FIG. 1, in some examples, video encoder 200 andvideo decoder 300 may each be integrated with an audio encoder and/oraudio decoder, and may include appropriate MUX-DEMUX units, or otherhardware and/or software, to handle multiplexed streams including bothaudio and video in a common data stream. If applicable, MUX-DEMUX unitsmay conform to the ITU H.223 multiplexer protocol, or other protocolssuch as the user datagram protocol (UDP).

Video encoder 200 and video decoder 300 each may be implemented as anyof a variety of suitable encoder and/or decoder circuitry, such as oneor more microprocessors, digital signal processors (DSPs), applicationspecific integrated circuits (ASICs), field programmable gate arrays(FPGAs), discrete logic, software, hardware, firmware or anycombinations thereof. When the techniques are implemented partially insoftware, a device may store instructions for the software in asuitable, non-transitory computer-readable medium and execute theinstructions in hardware using one or more processors to perform thetechniques of this disclosure. Each of video encoder 200 and videodecoder 300 may be included in one or more encoders or decoders, eitherof which may be integrated as part of a combined encoder/decoder (CODEC)in a respective device. A device including video encoder 200 and/orvideo decoder 300 may comprise an integrated circuit, a microprocessor,and/or a wireless communication device, such as a cellular telephone.

Video encoder 200 and video decoder 300 may operate according to a videocoding standard, such as ITU-T H.265, also referred to as HighEfficiency Video Coding (HEVC) or extensions thereto, such as themulti-view and/or scalable video coding extensions. Alternatively, videoencoder 200 and video decoder 300 may operate according to otherproprietary or industry standards, such as the Joint Exploration TestModel (JEM) or ITU-T H.266, also referred to as Versatile Video Coding(VVC). A recent draft of the VVC standard is described in Bross, et al.“Versatile Video Coding (Draft 5),” Joint Video Experts Team (JVET) ofITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 14^(th) Meeting: Geneva,CH, 19-27 March 2019, JVET-N1001-v8 (hereinafter “VVC Draft 5”). Thetechniques of this disclosure, however, are not limited to anyparticular coding standard.

In general, video encoder 200 and video decoder 300 may performblock-based coding of pictures. The term “block” generally refers to astructure including data to be processed (e.g., encoded, decoded, orotherwise used in the encoding and/or decoding process). For example, ablock may include a two-dimensional matrix of samples of luminanceand/or chrominance data. In general, video encoder 200 and video decoder300 may code video data represented in a YUV (e.g., Y, Cb, Cr) format.That is, rather than coding red, green, and blue (RGB) data for samplesof a picture, video encoder 200 and video decoder 300 may code luminanceand chrominance components, where the chrominance components may includeboth red hue and blue hue chrominance components. In some examples,video encoder 200 converts received RGB formatted data to a YUVrepresentation prior to encoding, and video decoder 300 converts the YUVrepresentation to the RGB format. Alternatively, pre- andpost-processing units (not shown) may perform these conversions.

This disclosure may generally refer to coding (e.g., encoding anddecoding) of pictures to include the process of encoding or decodingdata of the picture. Similarly, this disclosure may refer to coding ofblocks of a picture to include the process of encoding or decoding datafor the blocks, e.g., prediction and/or residual coding. An encodedvideo bitstream generally includes a series of values for syntaxelements representative of coding decisions (e.g., coding modes) andpartitioning of pictures into blocks. Thus, references to coding apicture or a block should generally be understood as coding values forsyntax elements forming the picture or block.

HEVC defines various blocks, including coding units (CUs), predictionunits (PUs), and transform units (TUs). According to HEVC, a video coder(such as video encoder 200) partitions a coding tree unit (CTU) into CUsaccording to a quadtree structure. That is, the video coder partitionsCTUs and CUs into four equal, non-overlapping squares, and each node ofthe quadtree has either zero or four child nodes. Nodes without childnodes may be referred to as “leaf nodes,” and CUs of such leaf nodes mayinclude one or more PUs and/or one or more TUs. The video coder mayfurther partition PUs and TUs. For example, in HEVC, a residual quadtree(RQT) represents partitioning of TUs. In HEVC, PUs representinter-prediction data, while TUs represent residual data. CUs that areintra-predicted include intra-prediction information, such as anintra-mode indication.

As another example, video encoder 200 and video decoder 300 may beconfigured to operate according to VVC. According to VVC, a video coder(such as video encoder 200) partitions a picture into a plurality ofcoding tree units (CTUs). Video encoder 200 may partition a CTUaccording to a tree structure, such as a quadtree-binary tree (QTBT)structure or Multi-Type Tree (MTT) structure. The QTBT structure removesthe concepts of multiple partition types, such as the separation betweenCUs, PUs, and TUs of HEVC. A QTBT structure includes two levels: a firstlevel partitioned according to quadtree partitioning, and a second levelpartitioned according to binary tree partitioning. A root node of theQTBT structure corresponds to a CTU. Leaf nodes of the binary treescorrespond to coding units (CUs).

In an MTT partitioning structure, blocks may be partitioned using aquadtree (QT) partition, a binary tree (BT) partition, and one or moretypes of triple tree (TT) (also called ternary tree (TT)) partitions. Atriple or ternary tree partition is a partition where a block is splitinto three sub-blocks. In some examples, a triple or ternary treepartition divides a block into three sub-blocks without dividing theoriginal block through the center. The partitioning types in MTT (e.g.,QT, BT, and TT), may be symmetrical or asymmetrical.

In some examples, video encoder 200 and video decoder 300 may use asingle QTBT or MTT structure to represent each of the luminance andchrominance components, while in other examples, video encoder 200 andvideo decoder 300 may use two or more QTBT or MTT structures, such asone QTBT/MTT structure for the luminance component and another QTBT/MTTstructure for both chrominance components (or two QTBT/MTT structuresfor respective chrominance components).

Video encoder 200 and video decoder 300 may be configured to usequadtree partitioning per HEVC, QTBT partitioning, MTT partitioning, orother partitioning structures. For purposes of explanation, thedescription of the techniques of this disclosure is presented withrespect to QTBT partitioning. However, it should be understood that thetechniques of this disclosure may also be applied to video codersconfigured to use quadtree partitioning, or other types of partitioningas well.

In some examples, a CTU includes a coding tree block (CTB) of lumasamples, two corresponding CTBs of chroma samples of a picture that hasthree sample arrays, or a CTB of samples of a monochrome picture or apicture that is coded using three separate color planes and syntaxstructures used to code the samples. A CTB may be an N×N block ofsamples for some value of N such that the division of a component intoCTBs is a partitioning. A component is an array or single sample fromone of the three arrays (luma and two chroma) that compose a picture in4:2:0, 4:2:2, or 4:4:4 color format or the array or a single sample ofthe array that compose a picture in monochrome format. In some examples,a coding block is an M×N block of samples for some values of M and Nsuch that a division of a CTB into coding blocks is a partitioning.

The blocks (e.g., CTUs or CUs) may be grouped in various ways in apicture. As one example, a brick may refer to a rectangular region ofCTU rows within a particular tile in a picture. A tile may be arectangular region of CTUs within a particular tile column and aparticular tile row in a picture. A tile column refers to a rectangularregion of CTUs having a height equal to the height of the picture and awidth specified by syntax elements (e.g., such as in a picture parameterset). A tile row refers to a rectangular region of CTUs having a heightspecified by syntax elements (e.g., such as in a picture parameter set)and a width equal to the width of the picture.

In some examples, a tile may be partitioned into multiple bricks, eachof which may include one or more CTU rows within the tile. A tile thatis not partitioned into multiple bricks may also be referred to as abrick. However, a brick that is a true subset of a tile may not bereferred to as a tile.

The bricks in a picture may also be arranged in a slice. A slice may bean integer number of bricks of a picture that may be exclusivelycontained in a single network abstraction layer (NAL) unit. In someexamples, a slice includes either a number of complete tiles or only aconsecutive sequence of complete bricks of one tile.

This disclosure may use “N×N” and “N by N” interchangeably to refer tothe sample dimensions of a block (such as a CU or other video block) interms of vertical and horizontal dimensions, e.g., 16×16 samples or 16by 16 samples. In general, a 16×16 CU will have 16 samples in a verticaldirection (y=16) and 16 samples in a horizontal direction (x=16).Likewise, an N×N CU generally has N samples in a vertical direction andN samples in a horizontal direction, where N represents a nonnegativeinteger value. The samples in a CU may be arranged in rows and columns.Moreover, CUs need not necessarily have the same number of samples inthe horizontal direction as in the vertical direction. For example, CUsmay comprise N×M samples, where M is not necessarily equal to N.

Video encoder 200 encodes video data for CUs representing predictionand/or residual information, and other information. The predictioninformation indicates how the CU is to be predicted in order to form aprediction block for the CU. The residual information generallyrepresents sample-by-sample differences between samples of the CU priorto encoding and the prediction block.

To predict a CU, video encoder 200 may generally form a prediction blockfor the CU through inter-prediction or intra-prediction.Inter-prediction generally refers to predicting the CU from data of apreviously coded picture, whereas intra-prediction generally refers topredicting the CU from previously coded data of the same picture. Toperform inter-prediction, video encoder 200 may generate the predictionblock using one or more motion vectors. Video encoder 200 may generallyperform a motion search to identify a reference block that closelymatches the CU, e.g., in terms of differences between the CU and thereference block. Video encoder 200 may calculate a difference metricusing a sum of absolute difference (SAD), sum of squared differences(SSD), mean absolute difference (MAD), mean squared differences (MSD),or other such difference calculations to determine whether a referenceblock closely matches the current CU. In some examples, video encoder200 may predict the current CU using uni-directional prediction orbi-directional prediction.

Some examples of VVC also provide an affine motion compensation mode,which may be considered an inter-prediction mode. In affine motioncompensation mode, video encoder 200 may determine two or more motionvectors that represent non-translational motion, such as zoom in or out,rotation, perspective motion, or other irregular motion types.

To perform intra-prediction, video encoder 200 may select anintra-prediction mode to generate the prediction block. Some examples ofVVC provide sixty-seven intra-prediction modes, including variousdirectional modes, as well as planar mode and DC mode. In general, videoencoder 200 selects an intra-prediction mode that describes neighboringsamples to a current block (e.g., a block of a CU) from which to predictsamples of the current block. Such samples may generally be above, aboveand to the left, or to the left of the current block in the same pictureas the current block, assuming video encoder 200 codes CTUs and CUs inraster scan order (left to right, top to bottom).

Video encoder 200 encodes data representing the prediction mode for acurrent block. For example, for inter-prediction modes, video encoder200 may encode data representing which of the various availableinter-prediction modes is used, as well as motion information for thecorresponding mode. For uni-directional or bi-directionalinter-prediction, for example, video encoder 200 may encode motionvectors using advanced motion vector prediction (AMVP) or merge mode.Video encoder 200 may use similar modes to encode motion vectors foraffine motion compensation mode.

Following prediction, such as intra-prediction or inter-prediction of ablock, video encoder 200 may calculate residual data for the block. Theresidual data, such as a residual block, represents sample by sampledifferences between the block and a prediction block for the block,formed using the corresponding prediction mode. Video encoder 200 mayapply one or more transforms to the residual block, to producetransformed data in a transform domain instead of the sample domain. Forexample, video encoder 200 may apply a discrete cosine transform (DCT),an integer transform, a wavelet transform, or a conceptually similartransform to residual video data. Additionally, video encoder 200 mayapply a secondary transform following the first transform, such as amode-dependent non-separable secondary transform (MDNSST), a signaldependent transform, a Karhunen-Loeve transform (KLT), or the like.Video encoder 200 produces transform coefficients following applicationof the one or more transforms.

As noted above, following any transforms to produce transformcoefficients, video encoder 200 may perform quantization of thetransform coefficients. Quantization generally refers to a process inwhich transform coefficients are quantized to possibly reduce the amountof data used to represent the transform coefficients, providing furthercompression. By performing the quantization process, video encoder 200may reduce the bit depth associated with some or all of the transformcoefficients. For example, video encoder 200 may round an n-bit valuedown to an m-bit value during quantization, where n is greater than m.In some examples, to perform quantization, video encoder 200 may performa bitwise right-shift of the value to be quantized.

Following quantization, video encoder 200 may scan the transformcoefficients, producing a one-dimensional vector from thetwo-dimensional matrix including the quantized transform coefficients.The scan may be designed to place higher energy (and therefore lowerfrequency) transform coefficients at the front of the vector and toplace lower energy (and therefore higher frequency) transformcoefficients at the back of the vector. In some examples, video encoder200 may utilize a predefined scan order to scan the quantized transformcoefficients to produce a serialized vector, and then entropy encode thequantized transform coefficients of the vector. In other examples, videoencoder 200 may perform an adaptive scan. After scanning the quantizedtransform coefficients to form the one-dimensional vector, video encoder200 may entropy encode the one-dimensional vector, e.g., according tocontext-adaptive binary arithmetic coding (CABAC). Video encoder 200 mayalso entropy encode values for syntax elements describing metadataassociated with the encoded video data for use by video decoder 300 indecoding the video data.

To perform CABAC, video encoder 200 may assign a context within acontext model to a symbol to be transmitted. The context may relate to,for example, whether neighboring values of the symbol are zero-valued ornot. The probability determination may be based on a context assigned tothe symbol.

Video encoder 200 may further generate syntax data, such as block-basedsyntax data, picture-based syntax data, and sequence-based syntax data,to video decoder 300, e.g., in a picture header, a block header, a sliceheader, or other syntax data, such as a sequence parameter set (SPS),picture parameter set (PPS), or video parameter set (VPS). Video decoder300 may likewise decode such syntax data to determine how to decodecorresponding video data.

In this manner, video encoder 200 may generate a bitstream includingencoded video data, e.g., syntax elements describing partitioning of apicture into blocks (e.g., CUs) and prediction and/or residualinformation for the blocks. Ultimately, video decoder 300 may receivethe bitstream and decode the encoded video data.

In general, video decoder 300 performs a reciprocal process to thatperformed by video encoder 200 to decode the encoded video data of thebitstream. For example, video decoder 300 may decode values for syntaxelements of the bitstream using CABAC in a manner substantially similarto, albeit reciprocal to, the CABAC encoding process of video encoder200. The syntax elements may define partitioning information forpartitioning of a picture into CTUs, and partitioning of each CTUaccording to a corresponding partition structure, such as a QTBTstructure, to define CUs of the CTU. The syntax elements may furtherdefine prediction and residual information for blocks (e.g., CUs) ofvideo data.

The residual information may be represented by, for example, quantizedtransform coefficients. Video decoder 300 may inverse quantize andinverse transform the quantized transform coefficients of a block toreproduce a residual block for the block. Video decoder 300 uses asignaled prediction mode (intra- or inter-prediction) and relatedprediction information (e.g., motion information for inter-prediction)to form a prediction block for the block. Video decoder 300 may thencombine the prediction block and the residual block (on asample-by-sample basis) to reproduce the original block. Video decoder300 may perform additional processing, such as performing a deblockingprocess to reduce visual artifacts along boundaries of the block.

In accordance with the techniques of this disclosure, video encoder 200may be configured to determine a first picture to remove from a DPB, andencode a first syntax element (e.g., a DPB index) indicating the firstpicture to remove from the DPB. Video encoder 200 may also encode andreconstruct a current picture, and store the reconstructed currentpicture in the DPB. In a reciprocal fashion, video decoder 300 may beconfigured to decode a first syntax element indicating a first pictureto remove from a DPB, remove the first picture from the DPB, decode acurrent picture, and store the decoded current picture in the DPB.

This disclosure may generally refer to “signaling” certain information,such as syntax elements. The term “signaling” may generally refer to thecommunication of values for syntax elements and/or other data used todecode encoded video data. That is, video encoder 200 may signal valuesfor syntax elements in the bitstream. In general, signaling refers togenerating a value in the bitstream. As noted above, source device 102may transport the bitstream to destination device 116 substantially inreal time, or not in real time, such as might occur when storing syntaxelements to storage device 112 for later retrieval by destination device116.

FIGS. 2A and 2B are conceptual diagram illustrating an example quadtreebinary tree (QTBT) structure 130, and a corresponding coding tree unit(CTU) 132. The solid lines represent quadtree splitting, and dottedlines indicate binary tree splitting. In each split (i.e., non-leaf)node of the binary tree, one flag is signaled to indicate whichsplitting type (i.e., horizontal or vertical) is used, where 0 indicateshorizontal splitting and 1 indicates vertical splitting in this example.For the quadtree splitting, there is no need to indicate the splittingtype, since quadtree nodes split a block horizontally and verticallyinto 4 sub-blocks with equal size. Accordingly, video encoder 200 mayencode, and video decoder 300 may decode, syntax elements (such assplitting information) for a region tree level of QTBT structure 130(i.e., the solid lines) and syntax elements (such as splittinginformation) for a prediction tree level of QTBT structure 130 (i.e.,the dashed lines). Video encoder 200 may encode, and video decoder 300may decode, video data, such as prediction and transform data, for CUsrepresented by terminal leaf nodes of QTBT structure 130.

In general, CTU 132 of FIG. 2B may be associated with parametersdefining sizes of blocks corresponding to nodes of QTBT structure 130 atthe first and second levels. These parameters may include a CTU size(representing a size of CTU 132 in samples), a minimum quadtree size(MinQTSize, representing a minimum allowed quadtree leaf node size), amaximum binary tree size (MaxBTSize, representing a maximum allowedbinary tree root node size), a maximum binary tree depth (MaxBTDepth,representing a maximum allowed binary tree depth), and a minimum binarytree size (MinBTSize, representing the minimum allowed binary tree leafnode size).

The root node of a QTBT structure corresponding to a CTU may have fourchild nodes at the first level of the QTBT structure, each of which maybe partitioned according to quadtree partitioning. That is, nodes of thefirst level are either leaf nodes (having no child nodes) or have fourchild nodes. The example of QTBT structure 130 represents such nodes asincluding the parent node and child nodes having solid lines forbranches. If nodes of the first level are not larger than the maximumallowed binary tree root node size (MaxBTSize), then the nodes can befurther partitioned by respective binary trees. The binary treesplitting of one node can be iterated until the nodes resulting from thesplit reach the minimum allowed binary tree leaf node size (MinBTSize)or the maximum allowed binary tree depth (MaxBTDepth). The example ofQTBT structure 130 represents such nodes as having dashed lines forbranches. The binary tree leaf node is referred to as a coding unit(CU), which is used for prediction (e.g., intra-picture or inter-pictureprediction) and transform, without any further partitioning. Asdiscussed above, CUs may also be referred to as “video blocks” or“blocks.”

In one example of the QTBT partitioning structure, the CTU size is setas 128×128 (luma samples and two corresponding 64×64 chroma samples),the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64, theMinBTSize (for both width and height) is set as 4, and the MaxBTDepth isset as 4. The quadtree partitioning is applied to the CTU first togenerate quad-tree leaf nodes. The quadtree leaf nodes may have a sizefrom 16×16 (i.e., the MinQTSize) to 128×128 (i.e., the CTU size). If theleaf quadtree node is 128×128, it will not be further split by thebinary tree, since the size exceeds the MaxBTSize (i.e., 64×64, in thisexample). Otherwise, the leaf quadtree node will be further partitionedby the binary tree. Therefore, the quadtree leaf node is also the rootnode for the binary tree and has the binary tree depth as 0. When thebinary tree depth reaches MaxBTDepth (4, in this example), no furthersplitting is permitted. When the binary tree node has width equal toMinBTSize (4, in this example), it implies no further horizontalsplitting is permitted. Similarly, a binary tree node having a heightequal to MinBTSize implies no further vertical splitting is permittedfor that binary tree node. As noted above, leaf nodes of the binary treeare referred to as CUs, and are further processed according toprediction and transform without further partitioning.

FIG. 3 is a block diagram illustrating an example video encoder 200 thatmay perform the techniques of this disclosure. FIG. 3 is provided forpurposes of explanation and should not be considered limiting of thetechniques as broadly exemplified and described in this disclosure. Forpurposes of explanation, this disclosure describes video encoder 200according to the techniques of VVC (ITU-T H.266, under development), andHEVC (ITU-T H.265). However, the techniques of this disclosure may beperformed by video encoding devices that are configured to other videocoding standards.

In the example of FIG. 3, video encoder 200 includes video data memory230, mode selection unit 202, residual generation unit 204, transformprocessing unit 206, quantization unit 208, inverse quantization unit210, inverse transform processing unit 212, reconstruction unit 214,filter unit 216, decoded picture buffer (DPB) 218, and entropy encodingunit 220. Any or all of video data memory 230, mode selection unit 202,residual generation unit 204, transform processing unit 206,quantization unit 208, inverse quantization unit 210, inverse transformprocessing unit 212, reconstruction unit 214, filter unit 216, DPB 218,and entropy encoding unit 220 may be implemented in one or moreprocessors or in processing circuitry. For instance, the units of videoencoder 200 may be implemented as one or more circuits or logic elementsas part of hardware circuitry, or as part of a processor, ASIC, of FPGA.Moreover, video encoder 200 may include additional or alternativeprocessors or processing circuitry to perform these and other functions.

Video data memory 230 may store video data to be encoded by thecomponents of video encoder 200. Video encoder 200 may receive the videodata stored in video data memory 230 from, for example, video source 104(FIG. 1). DPB 218 may act as a reference picture memory that storesreference video data for use in prediction of subsequent video data byvideo encoder 200. Video data memory 230 and DPB 218 may be formed byany of a variety of memory devices, such as dynamic random access memory(DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM),resistive RAM (RRAM), or other types of memory devices. Video datamemory 230 and DPB 218 may be provided by the same memory device orseparate memory devices. In various examples, video data memory 230 maybe on-chip with other components of video encoder 200, as illustrated,or off-chip relative to those components.

In this disclosure, reference to video data memory 230 should not beinterpreted as being limited to memory internal to video encoder 200,unless specifically described as such, or memory external to videoencoder 200, unless specifically described as such. Rather, reference tovideo data memory 230 should be understood as reference memory thatstores video data that video encoder 200 receives for encoding (e.g.,video data for a current block that is to be encoded). Memory 106 ofFIG. 1 may also provide temporary storage of outputs from the variousunits of video encoder 200.

The various units of FIG. 3 are illustrated to assist with understandingthe operations performed by video encoder 200. The units may beimplemented as fixed-function circuits, programmable circuits, or acombination thereof. Fixed-function circuits refer to circuits thatprovide particular functionality, and are preset on the operations thatcan be performed. Programmable circuits refer to circuits that can beprogrammed to perform various tasks, and provide flexible functionalityin the operations that can be performed. For instance, programmablecircuits may execute software or firmware that cause the programmablecircuits to operate in the manner defined by instructions of thesoftware or firmware. Fixed-function circuits may execute softwareinstructions (e.g., to receive parameters or output parameters), but thetypes of operations that the fixed-function circuits perform aregenerally immutable. In some examples, one or more of the units may bedistinct circuit blocks (fixed-function or programmable), and in someexamples, one or more of the units may be integrated circuits.

Video encoder 200 may include arithmetic logic units (ALUs), elementaryfunction units (EFUs), digital circuits, analog circuits, and/orprogrammable cores, formed from programmable circuits. In examples wherethe operations of video encoder 200 are performed using softwareexecuted by the programmable circuits, memory 106 (FIG. 1) may store theinstructions (e.g., object code) of the software that video encoder 200receives and executes, or another memory within video encoder 200 (notshown) may store such instructions.

Video data memory 230 is configured to store received video data. Videoencoder 200 may retrieve a picture of the video data from video datamemory 230 and provide the video data to residual generation unit 204and mode selection unit 202. Video data in video data memory 230 may beraw video data that is to be encoded.

Mode selection unit 202 includes a motion estimation unit 222, a motioncompensation unit 224, and an intra-prediction unit 226. Mode selectionunit 202 may include additional functional units to perform videoprediction in accordance with other prediction modes. As examples, modeselection unit 202 may include a palette unit, an intra-block copy unit(which may be part of motion estimation unit 222 and/or motioncompensation unit 224), an affine unit, a linear model (LM) unit, or thelike.

Mode selection unit 202 generally coordinates multiple encoding passesto test combinations of encoding parameters and resultingrate-distortion values for such combinations. The encoding parametersmay include partitioning of CTUs into CUs, prediction modes for the CUs,transform types for residual data of the CUs, quantization parametersfor residual data of the CUs, and so on. Mode selection unit 202 mayultimately select the combination of encoding parameters havingrate-distortion values that are better than the other testedcombinations.

Video encoder 200 may partition a picture retrieved from video datamemory 230 into a series of CTUs, and encapsulate one or more CTUswithin a slice. Mode selection unit 202 may partition a CTU of thepicture in accordance with a tree structure, such as the QTBT structureor the quad-tree structure of HEVC described above. As described above,video encoder 200 may form one or more CUs from partitioning a CTUaccording to the tree structure. Such a CU may also be referred togenerally as a “video block” or “block.”

In general, mode selection unit 202 also controls the components thereof(e.g., motion estimation unit 222, motion compensation unit 224, andintra-prediction unit 226) to generate a prediction block for a currentblock (e.g., a current CU, or in HEVC, the overlapping portion of a PUand a TU). For inter-prediction of a current block, motion estimationunit 222 may perform a motion search to identify one or more closelymatching reference blocks in one or more reference pictures (e.g., oneor more previously coded pictures stored in DPB 218). In particular,motion estimation unit 222 may calculate a value representative of howsimilar a potential reference block is to the current block, e.g.,according to sum of absolute difference (SAD), sum of squareddifferences (SSD), mean absolute difference (MAD), mean squareddifferences (MSD), or the like. Motion estimation unit 222 may generallyperform these calculations using sample-by-sample differences betweenthe current block and the reference block being considered. Motionestimation unit 222 may identify a reference block having a lowest valueresulting from these calculations, indicating a reference block thatmost closely matches the current block.

Motion estimation unit 222 may form one or more motion vectors (MVs)that defines the positions of the reference blocks in the referencepictures relative to the position of the current block in a currentpicture. Motion estimation unit 222 may then provide the motion vectorsto motion compensation unit 224. For example, for uni-directionalinter-prediction, motion estimation unit 222 may provide a single motionvector, whereas for bi-directional inter-prediction, motion estimationunit 222 may provide two motion vectors. Motion compensation unit 224may then generate a prediction block using the motion vectors. Forexample, motion compensation unit 224 may retrieve data of the referenceblock using the motion vector. As another example, if the motion vectorhas fractional sample precision, motion compensation unit 224 mayinterpolate values for the prediction block according to one or moreinterpolation filters. Moreover, for bi-directional inter-prediction,motion compensation unit 224 may retrieve data for two reference blocksidentified by respective motion vectors and combine the retrieved data,e.g., through sample-by-sample averaging or weighted averaging.

As another example, for intra-prediction, or intra-prediction coding,intra-prediction unit 226 may generate the prediction block from samplesneighboring the current block. For example, for directional modes,intra-prediction unit 226 may generally mathematically combine values ofneighboring samples and populate these calculated values in the defineddirection across the current block to produce the prediction block. Asanother example, for DC mode, intra-prediction unit 226 may calculate anaverage of the neighboring samples to the current block and generate theprediction block to include this resulting average for each sample ofthe prediction block.

Mode selection unit 202 provides the prediction block to residualgeneration unit 204. Residual generation unit 204 receives a raw,unencoded version of the current block from video data memory 230 andthe prediction block from mode selection unit 202. Residual generationunit 204 calculates sample-by-sample differences between the currentblock and the prediction block. The resulting sample-by-sampledifferences define a residual block for the current block. In someexamples, residual generation unit 204 may also determine differencesbetween sample values in the residual block to generate a residual blockusing residual differential pulse code modulation (RDPCM). In someexamples, residual generation unit 204 may be formed using one or moresubtractor circuits that perform binary subtraction.

In examples where mode selection unit 202 partitions CUs into PUs, eachPU may be associated with a luma prediction unit and correspondingchroma prediction units. Video encoder 200 and video decoder 300 maysupport PUs having various sizes. As indicated above, the size of a CUmay refer to the size of the luma coding block of the CU and the size ofa PU may refer to the size of a luma prediction unit of the PU. Assumingthat the size of a particular CU is 2N×2N, video encoder 200 may supportPU sizes of 2N×2N or N×N for intra prediction, and symmetric PU sizes of2N×2N, 2N×N, N×2N, N×N, or similar for inter prediction. Video encoder200 and video decoder 300 may also support asymmetric partitioning forPU sizes of 2N×nU, 2N×nD, nL×2N, and nR×2N for inter prediction.

In examples where mode selection unit 202 does not further partition aCU into PUs, each CU may be associated with a luma coding block andcorresponding chroma coding blocks. As above, the size of a CU may referto the size of the luma coding block of the CU. The video encoder 200and video decoder 300 may support CU sizes of 2N×2N, 2N×N, or N×2N.

For other video coding techniques such as an intra-block copy modecoding, an affine-mode coding, and linear model (LM) mode coding, as afew examples, mode selection unit 202, via respective units associatedwith the coding techniques, generates a prediction block for the currentblock being encoded. In some examples, such as palette mode coding, modeselection unit 202 may not generate a prediction block, and insteadgenerate syntax elements that indicate the manner in which toreconstruct the block based on a selected palette. In such modes, modeselection unit 202 may provide these syntax elements to entropy encodingunit 220 to be encoded.

As described above, residual generation unit 204 receives the video datafor the current block and the corresponding prediction block. Residualgeneration unit 204 then generates a residual block for the currentblock. To generate the residual block, residual generation unit 204calculates sample-by-sample differences between the prediction block andthe current block.

Transform processing unit 206 applies one or more transforms to theresidual block to generate a block of transform coefficients (referredto herein as a “transform coefficient block”). Transform processing unit206 may apply various transforms to a residual block to form thetransform coefficient block. For example, transform processing unit 206may apply a discrete cosine transform (DCT), a directional transform, aKarhunen-Loeve transform (KLT), or a conceptually similar transform to aresidual block. In some examples, transform processing unit 206 mayperform multiple transforms to a residual block, e.g., a primarytransform and a secondary transform, such as a rotational transform. Insome examples, transform processing unit 206 does not apply transformsto a residual block.

Quantization unit 208 may quantize the transform coefficients in atransform coefficient block, to produce a quantized transformcoefficient block. Quantization unit 208 may quantize transformcoefficients of a transform coefficient block according to aquantization parameter (QP) value associated with the current block.Video encoder 200 (e.g., via mode selection unit 202) may adjust thedegree of quantization applied to the transform coefficient blocksassociated with the current block by adjusting the QP value associatedwith the CU. Quantization may introduce loss of information, and thus,quantized transform coefficients may have lower precision than theoriginal transform coefficients produced by transform processing unit206.

Inverse quantization unit 210 and inverse transform processing unit 212may apply inverse quantization and inverse transforms to a quantizedtransform coefficient block, respectively, to reconstruct a residualblock from the transform coefficient block. Reconstruction unit 214 mayproduce a reconstructed block corresponding to the current block (albeitpotentially with some degree of distortion) based on the reconstructedresidual block and a prediction block generated by mode selection unit202. For example, reconstruction unit 214 may add samples of thereconstructed residual block to corresponding samples from theprediction block generated by mode selection unit 202 to produce thereconstructed block.

Filter unit 216 may perform one or more filter operations onreconstructed blocks. For example, filter unit 216 may performdeblocking operations to reduce blockiness artifacts along edges of CUs.Operations of filter unit 216 may be skipped, in some examples.

Video encoder 200 stores reconstructed blocks in DPB 218. For instance,in examples where operations of filter unit 216 are not needed,reconstruction unit 214 may store reconstructed blocks to DPB 218. Inexamples where operations of filter unit 216 are needed, filter unit 216may store the filtered reconstructed blocks to DPB 218. Motionestimation unit 222 and motion compensation unit 224 may retrieve areference picture from DPB 218, formed from the reconstructed (andpotentially filtered) blocks, to inter-predict blocks of subsequentlyencoded pictures. In addition, intra-prediction unit 226 may usereconstructed blocks in DPB 218 of a current picture to intra-predictother blocks in the current picture.

In accordance with the techniques of the disclosure, as will beexplained in more detail below, video encoder 200 may be configured todetermine a first picture to remove from DPB 218 and encode a firstsyntax element (e.g., a DPB index) indicating the first picture toremove from a DPB at video decoder 300. Video encoder 200 may signal thefirst syntax element in an encoded video bitstream to video decoder 300.Video encoder 200 may also encode and reconstruct a current picture, andstore the reconstructed current picture in DPB 218.

In general, entropy encoding unit 220 may entropy encode syntax elementsreceived from other functional components of video encoder 200. Forexample, entropy encoding unit 220 may entropy encode quantizedtransform coefficient blocks from quantization unit 208. As anotherexample, entropy encoding unit 220 may entropy encode prediction syntaxelements (e.g., motion information for inter-prediction or intra-modeinformation for intra-prediction) from mode selection unit 202. Entropyencoding unit 220 may perform one or more entropy encoding operations onthe syntax elements, which are another example of video data, togenerate entropy-encoded data. For example, entropy encoding unit 220may perform a context-adaptive variable length coding (CAVLC) operation,a CABAC operation, a variable-to-variable (V2V) length coding operation,a syntax-based context-adaptive binary arithmetic coding (SBAC)operation, a Probability Interval Partitioning Entropy (PIPE) codingoperation, an Exponential-Golomb encoding operation, or another type ofentropy encoding operation on the data. In some examples, entropyencoding unit 220 may operate in bypass mode where syntax elements arenot entropy encoded.

Video encoder 200 may output a bitstream that includes the entropyencoded syntax elements needed to reconstruct blocks of a slice orpicture. In particular, entropy encoding unit 220 may output thebitstream.

The operations described above are described with respect to a block.Such description should be understood as being operations for a lumacoding block and/or chroma coding blocks. As described above, in someexamples, the luma coding block and chroma coding blocks are luma andchroma components of a CU. In some examples, the luma coding block andthe chroma coding blocks are luma and chroma components of a PU.

In some examples, operations performed with respect to a luma codingblock need not be repeated for the chroma coding blocks. As one example,operations to identify a motion vector (MV) and reference picture for aluma coding block need not be repeated for identifying an MV andreference picture for the chroma blocks. Rather, the MV for the lumacoding block may be scaled to determine the MV for the chroma blocks,and the reference picture may be the same. As another example, theintra-prediction process may be the same for the luma coding block andthe chroma coding blocks.

Video encoder 200 represents an example of a device configured to encodevideo data. Video encoder 200 may, for example, include a memoryconfigured to store video data and include one or more processing unitsimplemented in circuitry and configured to determine a first picture toremove from a DPB, and encode a first syntax element (e.g., a DPB index)indicating the first picture to remove from the DPB. The first syntaxelement may be encoded in an encoded video bitstream. Video encoder 200may also encode and reconstruct a current picture, and store thereconstructed current picture in the DPB.

FIG. 4 is a block diagram illustrating an example video decoder 300 thatmay perform the techniques of this disclosure. FIG. 4 is provided forpurposes of explanation and is not limiting on the techniques as broadlyexemplified and described in this disclosure. For purposes ofexplanation, this disclosure describes video decoder 300 according tothe techniques of VVC (ITU-T H.266, under development), and HEVC (ITU-TH.265). However, the techniques of this disclosure may be performed byvideo coding devices that are configured to other video codingstandards.

In the example of FIG. 4, video decoder 300 includes coded picturebuffer (CPB) memory 320, entropy decoding unit 302, predictionprocessing unit 304, inverse quantization unit 306, inverse transformprocessing unit 308, reconstruction unit 310, filter unit 312, anddecoded picture buffer (DPB) 314. Any or all of CPB memory 320, entropydecoding unit 302, prediction processing unit 304, inverse quantizationunit 306, inverse transform processing unit 308, reconstruction unit310, filter unit 312, and DPB 314 may be implemented in one or moreprocessors or in processing circuitry. For instance, the units of videodecoder 300 may be implemented as one or more circuits or logic elementsas part of hardware circuitry, or as part of a processor, ASIC, of FPGA.Moreover, video decoder 300 may include additional or alternativeprocessors or processing circuitry to perform these and other functions.

Prediction processing unit 304 includes motion compensation unit 316 andintra-prediction unit 318. Prediction processing unit 304 may includeadditional units to perform prediction in accordance with otherprediction modes. As examples, prediction processing unit 304 mayinclude a palette unit, an intra-block copy unit (which may form part ofmotion compensation unit 316), an affine unit, a linear model (LM) unit,or the like. In other examples, video decoder 300 may include more,fewer, or different functional components.

CPB memory 320 may store video data, such as an encoded video bitstream,to be decoded by the components of video decoder 300. The video datastored in CPB memory 320 may be obtained, for example, fromcomputer-readable medium 110 (FIG. 1). CPB memory 320 may include a CPBthat stores encoded video data (e.g., syntax elements) from an encodedvideo bitstream. Also, CPB memory 320 may store video data other thansyntax elements of a coded picture, such as temporary data representingoutputs from the various units of video decoder 300. DPB 314 generallystores decoded pictures, which video decoder 300 may output and/or useas reference video data when decoding subsequent data or pictures of theencoded video bitstream. CPB memory 320 and DPB 314 may be formed by anyof a variety of memory devices, such as DRAM, including SDRAM, MRAM,RRAM, or other types of memory devices. CPB memory 320 and DPB 314 maybe provided by the same memory device or separate memory devices. Invarious examples, CPB memory 320 may be on-chip with other components ofvideo decoder 300, or off-chip relative to those components.

Additionally or alternatively, in some examples, video decoder 300 mayretrieve coded video data from memory 120 (FIG. 1). That is, memory 120may store data as discussed above with CPB memory 320. Likewise, memory120 may store instructions to be executed by video decoder 300, whensome or all of the functionality of video decoder 300 is implemented insoftware to be executed by processing circuitry of video decoder 300.

The various units shown in FIG. 4 are illustrated to assist withunderstanding the operations performed by video decoder 300. The unitsmay be implemented as fixed-function circuits, programmable circuits, ora combination thereof. Similar to FIG. 3, fixed-function circuits referto circuits that provide particular functionality, and are preset on theoperations that can be performed. Programmable circuits refer tocircuits that can be programmed to perform various tasks, and provideflexible functionality in the operations that can be performed. Forinstance, programmable circuits may execute software or firmware thatcause the programmable circuits to operate in the manner defined byinstructions of the software or firmware. Fixed-function circuits mayexecute software instructions (e.g., to receive parameters or outputparameters), but the types of operations that the fixed-functioncircuits perform are generally immutable. In some examples, one or moreof the units may be distinct circuit blocks (fixed-function orprogrammable), and in some examples, one or more of the units may beintegrated circuits.

Video decoder 300 may include ALUs, EFUs, digital circuits, analogcircuits, and/or programmable cores formed from programmable circuits.In examples where the operations of video decoder 300 are performed bysoftware executing on the programmable circuits, on-chip or off-chipmemory may store instructions (e.g., object code) of the software thatvideo decoder 300 receives and executes.

Entropy decoding unit 302 may receive encoded video data from the CPBand entropy decode the video data to reproduce syntax elements.Prediction processing unit 304, inverse quantization unit 306, inversetransform processing unit 308, reconstruction unit 310, and filter unit312 may generate decoded video data based on the syntax elementsextracted from the bitstream.

In general, video decoder 300 reconstructs a picture on a block-by-blockbasis. Video decoder 300 may perform a reconstruction operation on eachblock individually (where the block currently being reconstructed, i.e.,decoded, may be referred to as a “current block”).

Entropy decoding unit 302 may entropy decode syntax elements definingquantized transform coefficients of a quantized transform coefficientblock, as well as transform information, such as a quantizationparameter (QP) and/or transform mode indication(s). Inverse quantizationunit 306 may use the QP associated with the quantized transformcoefficient block to determine a degree of quantization and, likewise, adegree of inverse quantization for inverse quantization unit 306 toapply. Inverse quantization unit 306 may, for example, perform a bitwiseleft-shift operation to inverse quantize the quantized transformcoefficients. Inverse quantization unit 306 may thereby form a transformcoefficient block including transform coefficients.

After inverse quantization unit 306 forms the transform coefficientblock, inverse transform processing unit 308 may apply one or moreinverse transforms to the transform coefficient block to generate aresidual block associated with the current block. For example, inversetransform processing unit 308 may apply an inverse DCT, an inverseinteger transform, an inverse Karhunen-Loeve transform (KLT), an inverserotational transform, an inverse directional transform, or anotherinverse transform to the transform coefficient block.

Furthermore, prediction processing unit 304 generates a prediction blockaccording to prediction information syntax elements that were entropydecoded by entropy decoding unit 302. For example, if the predictioninformation syntax elements indicate that the current block isinter-predicted, motion compensation unit 316 may generate theprediction block. In this case, the prediction information syntaxelements may indicate a reference picture in DPB 314 from which toretrieve a reference block, as well as a motion vector identifying alocation of the reference block in the reference picture relative to thelocation of the current block in the current picture. Motioncompensation unit 316 may generally perform the inter-prediction processin a manner that is substantially similar to that described with respectto motion compensation unit 224 (FIG. 3).

As another example, if the prediction information syntax elementsindicate that the current block is intra-predicted, intra-predictionunit 318 may generate the prediction block according to anintra-prediction mode indicated by the prediction information syntaxelements. Intra-prediction unit 318 may generally perform theintra-prediction process in a manner that is substantially similar tothat described with respect to intra-prediction unit 226 (FIG. 3).Intra-prediction unit 318 may retrieve data of neighboring samples tothe current block from DPB 314.

Reconstruction unit 310 may reconstruct the current block using theprediction block and the residual block. For example, reconstructionunit 310 may add samples of the residual block to corresponding samplesof the prediction block to reconstruct the current block.

Filter unit 312 may perform one or more filter operations onreconstructed blocks. For example, filter unit 312 may performdeblocking operations to reduce blockiness artifacts along edges of thereconstructed blocks. Operations of filter unit 312 are not necessarilyperformed in all examples.

Video decoder 300 may store the reconstructed blocks in DPB 314. Forinstance, in examples where operations of filter unit 312 are notperformed, reconstruction unit 310 may store reconstructed blocks to DPB314. In examples where operations of filter unit 312 are performed,filter unit 312 may store the filtered reconstructed blocks to DPB 314.As discussed above, DPB 314 may provide reference information, such assamples of a current picture for intra-prediction and previously decodedpictures for subsequent motion compensation, to prediction processingunit 304. Moreover, video decoder 300 may output decoded pictures (e.g.,decoded video) from DPB 314 for subsequent presentation on a displaydevice, such as display device 118 of FIG. 1.

In accordance with the techniques of this disclosure, as will bedescribed in more detail below, video decoder 300 may be configured todecode a syntax element (e.g., a DPB index) that indicates a picture toremove from DPB 314. In response to the value of the syntax element,video decoder 300 may be configured to remove a picture from DPB 314that is associated with the value of the syntax element. Video decoder300 may then decode a current picture, store the decoded current picturein DPB 314.

In HEVC and VVC, to indicate which pictures can be removed from thedecoded picture buffer (DPB), video encoder 200 and video decoder 300may perform a marking process on pictures included in a referencepicture list. For example, video encoder 200 and video decoder 300 maybe configured to mark pictures (e.g., perform a marking process) notincluded in one or more reference picture lists (e.g., reference picturelist 0 and reference picture list 1) as “unused for reference.” Videoencoder 200 and video decoder 300 may be configured to remove picturesmarked as “unused for reference” and not needed for output from the DPB.This is sometimes referred to as a bumping process. The bumping processis needed to free memory in the DPB in order to store the currentlydecoded picture.

In some examples, video encoder 200 and video decoder 300 may beconfigured to invoke the picture marking process for the first slice ina picture. For this reason, HEVC has defined a constraint that thecontent for the reference picture lists in all slices of the pictureshall be the same. That is, every slice is the picture must have thesame pictures in the reference picture list.

The above marking and bumping techniques exhibit several drawbacks. Forexample, video encoder 200 and video decoder 300, if coding video datain accordance with HEVC, may perform a picture marking process for allpictures included in a reference picture list to free space in the DPBfor the currently decoded picture. However, based on the aboveconstraint, different slices in the same picture cannot have referencepicture lists having different reference pictures due to the fact thatthe marking process is invoked only for the first slice in a picture andbumping (e.g., removing pictures from the DPB) is based on this marking.

Reference Picture List Constraint

In one example of the disclosure, video encoder 200 and video decoder300 are configured to perform a marking and bumping process for a DPBbased on a relaxed constraint relative to the HEVC constraint describedabove. In one example, video encoder 200 and video decoder 300 areconfigured to perform a marking and bumping process under the constraintthat only the first slice in a picture shall include all the pictures inthe reference picture lists (either in RefPicList0, RefPicList1, or inboth) that are used as references in all slices of the same picture. Inthis case, video encoder 200 and video decoder 300 may perform a markingprocesses for these reference picture lists and all reference picturesneeded for all slices in a picture will be preserved in the DPB (e.g.,DPB 218 and/or DPB 314).

A potential advantage of such approach is that subsequent slices (secondslices and later in coding order) in the same picture may have shorterreference picture lists. This may result in less overhead to signal,rather than signaling all reference pictures in each slice of thepicture.

The following examples of the disclosure may be described with referenceto video encoder 200 signaling various syntax elements. It should beunderstood that the following examples further cover video decoder 300receiving, parsing, and decoding such syntax elements, as well asperforming any decoding processes in accordance with the values of thesyntax elements. Furthermore, in general, the term “signaling” a syntaxelement may refer to encoding the syntax element. The term “coding” maygenerically refer to both encoding and decoding.

DPB Indexing

In another example of the disclosure, video encoder 200 and videodecoder 300 do not use a reference picture list for a bumping process.As described above, to decode or reconstruct a picture, there may be aneed to release space in DPB 218 and/or DPB 314 to store this currentlyreconstructed/decoded picture. In this example of the disclosure, videoencoder 200 may be configured to explicitly indicate, e.g., with asyntax element, which picture and/or which memory locations in DPB 218and/or DPB 314 to release (i.e., the picture to remove from the DPBand/or the memory locations to be marked as free to use).

In general, in one example of the disclosure, video encoder 200 may beconfigured to encode and signal a first syntax element that indicateswhich picture in a DPB may be removed. Video decoder 300 may beconfigured to decode the first syntax element that indicates the pictureto remove from the DPB, and may remove the picture from the DPB.Removing a picture from a DPB may include overwriting the picture fromthe DPB, marking the portions of memory currently storing the picture tobe removed as available for overwriting, deleting the data in theportions of memory currently storing the picture to be removed, oranother other technique for freeing the memory space for future writesof data. Video decoder 300 may further be configured to decode a currentpicture, and store the decoded current picture in the DPB.

For example, video encoder 200 may signal an index (e.g., dpb_idx) thatindicates which picture in the DPB can be released or replaced by thecurrent picture. In one example, a picture in the DPB that may not beused for reference, but is still needed for output shall not beindicated by dpb_idx. That is, video encoder 200 may not signal thedpb_idx for any picture that is still needed for output, even if such apicture is not used reference. Accordingly, in the general exampleabove, the first syntax element is a DPB index (dpb_idx), and whereineach value of the DPB index is associated with a particular picture inthe decoded picture buffer.

For example, if the DPB size is 6 pictures, the dpb_idx can have valuesfrom 0 to 5. In this example, if video encoder 200 signals a dpb_idxvalue equal to 2, video decoder 300 decodes the dpb_idx and thenreplaces and/or bumps the third picture in the DPB based on the value of2 for dpb_idx. Other pictures may stay in the DPB.

Signaling which pictures to keep in the DPB by including such picturesin a reference picture list has associated overhead, since some picturesare just signaled in the reference picture list of a current picture tobe kept for the subsequent pictures (e.g., carried over), even if suchpictures are not used for inter prediction of the current picture. Inthe techniques of this disclosure, a video encoder may explicitly signalan index (e.g., a DPB index) to a picture that can be replaced or bumpedfrom the DPB. In this way, only the pictures actually used for referenceneed to be signaled in the reference picture list. As such, overheadsignaling is reduced and coding efficiency is increased.

Video encoder 200 may be configured to signal the DPB index (dpb_idx) inany parameter set, slice header, tile header, or elsewhere. Depending onthe syntax structure where the DPB index (dpb_idx) is signaled, videoencoder 200 and video decoder 300 may be configured to operate accordingto a constraint that the signaled DPB index (dpb_idx) shall be the samewithin the same picture, since this picture will be placed in the DPBonce reconstructed/decoded and only one picture currently in the DPB isneeded to be bumped. For example, if the DPB index (dpb_idx) is signaledin a slice header, then the constraint can be expressed such that allslices of the same picture shall have the same value of the DPB index(dpb_idx).

In one example, when configured to use a DPB index (dpb_idx) asdescribed above, video encoder 200 and video decoder 300 may code thevalue of the DPB index (dpb_idx) using a truncated binarization, sincethe DPB size is known and the maximum value of the DPB index (dpb_idx)is equal to the DPB size minus 1. In one example, video encoder 200 andvideo decoder 300 may code the value of the DPB index (dpb_idx) using atruncated binary code.

In other examples of this disclosure, for some pictures, video encoder200 may encode and signal multiple DPB indices (e.g., multiple values ofdpb_idx) to indicate which of a plurality of pictures to bump (e.g.,remove) from the DPB if there is a need for additional memory space.

In some example, video encoder 200 and video decoder 300 are configuredto code the index of the picture to be removed from the DPB using a DPBindex syntax element called dpb_idx_plus1. When the value ofdpb_idx_plus1 is greater than 0, dpb_idx_plus1 minus 1 specifies theindex to the picture in the DPB that is to be removed. A value ofdpb_idx_plus1 equal to 0 specifies that no picture in the DPB is to beremoved. The value of dpb_idx_plus1 shall be in the range of 0 toNumBufsInDpb, inclusive, where NumBufsInDpb specifies a number ofpicture buffers in the DPB. The value of NumBufsInDpb may be equal tothe maximum number of buffers that may be stored in the DPB based on oneor more of the following: the picture size, the DPB size, temporal IDsused, etc. In other examples, video encoder 200 and video decoder 300may operate according to an additional constraint to ensure that thevalue of NumBufsInDpb does not exceed a value derived for the DPB for aparticular operating point of video decoder 300 (e.g., based on picturesize, temporal ID, profile, tier, level, etc.).

In some examples, video encoder 200 may specify the picture to beremoved using a value other than a DPB index. For example, video encoder200 may specify the picture to be removed using a value related to thepicture order count (POC) value of the picture to be removed. Forexample, such a value may be the POC least significant bit (LSB) of thepicture to be removed, a deltaPOC value from the current picture (e.g.,the difference between the POC of the current picture and the POC of thepicture to be removed), a value derived from POC LSB of the picture toremoved, or a combination thereof.

Multiple Picture Removals

In some examples, video encoder 200 may be configured to signal syntaxelements indicating that multiple DPB pictures are to be removed. Thismay occur when there are sudden changes in the number of buffers thatthe DPB may store (e.g., when a picture resolution change occurs). Videoencoder 200 may signal a number of DPB index values (e.g., dpb_idx[ ]values). Video encoder 200 may also signal a syntax element thatindicates the number of DPB indices that will be signaled (e.g.,NumDpbIdxsToRemove). Video encoder 200 may signal a value of dpb_idx[i]for i in the range of 0 to NumDpbIdxsToRemove−1, inclusive.

In some examples, the dpb_idx[i] refer to the indices of the pictures inthe DPB before any removal has occurred for the current picture. Inother examples, dpb_idx[0] refers to the indices before any removal hasoccurred for the current picture, and dpb_idx[i] refers to the indicesafter the removal of the picture as indicated by dpb_idx[j] from j=0 . .. i−1, inclusive.

In some examples, one of the buffers in the DPB indicated by one ofdpb_idx[i] may be used to store the current picture. This value may bepredetermined (e.g., i=0) or may be determined by other syntax elementsin the bitstream.

In some examples, when the removal of a picture specified by a DPB indexis followed by inserting a picture (e.g., current picture) in thatlocation in the DPB, the indices of the rest of the pictures in thebuffer may not change.

Picture Marking

Additionally, in a further example of the disclosure, a picture markingprocess (e.g., marking as “used for reference” or “unused forreference”) can be removed, since it is known which picture to bump asindicated by dpb_idx. That is, video decoder 300 is configured to notperform a picture marking process.

In yet another example of the disclosure, the picture marking processfor marking pictures as “used for short-term reference” and “used forlong-term reference” may be also removed. However, in some video codecs,if a reference picture is marked as a long term reference picture, thenmotion vector scaling is typically not performed. To retain thisfunctionality of motion vector scaling in the case that picture markingis removed, video encoder 200 may signal a flag indicating whether amotion vector scaling is performed for a reference picture to which themotion vector points.

Zero Number of Entries in the Reference Picture List

In some examples, it is possible to have the size of the referencepicture list be equal to 0 for all slice types. In this case, no interprediction can be used even for non-I slices. A non-I slice is a slicewhere inter prediction is typically allowed. In an I slice, only intraprediction is performed. However, inter prediction related syntaxsignaling, such as a skip flag, merge index, motion vector predictor(MVP) flag, adaptive motion vector resolution (AMVR) flag, interprediction direction index (e.g., inter_pred_idc (interDir)), and anaffine flag (e.g., inter_affine_flag_, prediction mode flag, and othersuch syntax elements) may be signaled, even if not used.

In one example of the disclosure, video encoder 200 may conditionallysignal inter prediction related signaling when the reference picturelist size is 0, excluding intra block copy (IBC) related syntaxsignaling, which shall be allowed.

In some examples, when the reference picture list size is 0 then:

-   -   The skip flag (cu_skip_flag) is signaled only for the block        sizes for which IBC mode is allowed and when the high level tool        flag (e.g., sps_ibc_enabled_flag) is enabled. For example, for        block sizes from 4×4 to 64×64, video encoder 200 signals the        cu_skip_flag. The cu_skip_flag is not signaled for 128×N and        N×128 blocks.    -   The IBC prediction mode flag (pred_mode_ibc_flag) is not        signaled, and if inter mode is signaled, the prediction mode        flag is inferred to be IBC. In another example, the IBC        prediction mode flag (pred_mode_ibc_flag) is signaled to        differentiate intra and IBC modes.    -   One or more merge flags (e.g., general_merge_flag or merge_flag)        may be signaled for the blocks when IBC mode is allowed and when        IBC high-level syntax (HLS) flag (e.g., sps_ibc_enabled_flag) is        enabled.

In another example, video encoder 200 and video decoder 300 may beconfigured to operate according to a constraint that, for non-I-slices,the reference picture list of size 0 cannot be signaled. In anotherexample of the constraint, the reference picture list size of 0 shallnot be used for P- and B-slices (however, signaling size 0 is possible).The size of the reference picture list may represent the size of theactive reference picture list, i.e., the list of the actually usedreference pictures in a slice.

In addition, for those slices, the reference picture list size minus 1is signaled, since a size of 0 is not possible. In other words,reference picture list size signaling depends on the slice type.

FIG. 5 is a flowchart illustrating an example method for encoding acurrent block. The current block may comprise a current CU. Althoughdescribed with respect to video encoder 200 (FIGS. 1 and 3), it shouldbe understood that other devices may be configured to perform a methodsimilar to that of FIG. 5.

In this example, video encoder 200 initially predicts the current block(350). For example, video encoder 200 may form a prediction block forthe current block. Video encoder 200 may then calculate a residual blockfor the current block (352). To calculate the residual block, videoencoder 200 may calculate a difference between the original, unencodedblock and the prediction block for the current block. Video encoder 200may then transform and quantize coefficients of the residual block(354). Next, video encoder 200 may scan the quantized transformcoefficients of the residual block (356). During the scan, or followingthe scan, video encoder 200 may entropy encode the coefficients (358).For example, video encoder 200 may encode the coefficients using CAVLCor CABAC. Video encoder 200 may then output the entropy coded data ofthe block (360).

FIG. 6 is a flowchart illustrating an example method for decoding acurrent block of video data. The current block may comprise a currentCU. Although described with respect to video decoder 300 (FIGS. 1 and4), it should be understood that other devices may be configured toperform a method similar to that of FIG. 6.

Video decoder 300 may receive entropy coded data for the current block,such as entropy coded prediction information and entropy coded data forcoefficients of a residual block corresponding to the current block(370). Video decoder 300 may entropy decode the entropy coded data todetermine prediction information for the current block and to reproducecoefficients of the residual block (372). Video decoder 300 may predictthe current block (374), e.g., using an intra- or inter-prediction modeas indicated by the prediction information for the current block, tocalculate a prediction block for the current block. Video decoder 300may then inverse scan the reproduced coefficients (376), to create ablock of quantized transform coefficients. Video decoder 300 may theninverse quantize and inverse transform the coefficients to produce aresidual block (378). Video decoder 300 may ultimately decode thecurrent block by combining the prediction block and the residual block(380).

FIG. 7 is a flowchart illustrating an example decoded picture buffermanagement method at video encoder 200 according to the techniques ofthe disclosure. In one example of the disclosure, video encoder 200 maybe configured to determine a first picture to remove from a DPB, e.g.,DPB 218 of FIG. 3 (400). Video encoder 200 may encode a first syntaxelement indicating the first picture to remove from the DPB (402). Videoencoder 200 may encode and reconstruct a current picture (404), andstore the reconstructed current picture in the DPB (406).

In one example, the first syntax element is a DPB index (dpb_idx),wherein each value of the DPB index is associated with a particularpicture in the decoded picture buffer. In another example, the firstsyntax element is a DPB index plus 1 (dpb_idx_plus1). Based on a valuefor dpb_idx_plus1 being greater than 0, dpb_idx_plus1 minus 1 specifiesan index to a particular picture in the decoded picture buffer that isto be removed. Based on the value of dpb_idx_plus1 being equal to 0, nopicture in the decoded picture buffer is to be removed.

In another example of the disclosure, the first syntax element indicatesthe first picture to remove from the DPB using a value related to apicture order count (POC) value of the first picture.

In another example of the disclosure, the first picture is not used forreference and is not needed for output. That is, video encoder 200 willnot indicate the first picture to be removed from the DPB if the firstpicture is still needed for output.

In one example of the disclosure, video encoder 200 may be configured toencode the first syntax element using a truncated binarization.

In another example of the disclosure, video encoder 200 may beconfigured to indicate multiple pictures to remove from a DPB. In thisexample, video encoder 200 may encode a second syntax element indicatinga number of pictures to remove from the DPB, and may encode a respectivefirst syntax element for each of the number of pictures, the respectivefirst syntax elements indicating a respective picture to remove from theDPB.

FIG. 8 is a flowchart illustrating an example decoded picture buffermanagement method at a video decoder according to the techniques of thedisclosure. In one example of the disclosure, video decoder 300 may beconfigured to decode a first syntax element indicating a first pictureto remove from a DPB (450), and may remove the first picture from theDPB (452). Video decoder 300 may further decode a current picture (454),and store the decoded current picture in the DPB (456).

In one example, the first syntax element is a DPB index (dpb_idx),wherein each value of the DPB index is associated with a particularpicture in the decoded picture buffer. In another example, the firstsyntax element is a DPB index plus 1 (dpb_idx_plus1). Based on a valuefor dpb_idx_plus1 being greater than 0, dpb_idx_plus1 minus 1 specifiesan index to a particular picture in the decoded picture buffer that isto be removed. Based on the value of dpb_idx_plus1 being equal to 0, nopicture in the decoded picture buffer is to be removed.

In another example of the disclosure, the first syntax element indicatesthe first picture to remove from the DPB using a value related to apicture order count (POC) value of the first picture.

In another example of the disclosure, the first picture is not used forreference and is not needed for output. That is, video encoder 200 willnot indicate the first picture to be removed from the DPB if the firstpicture is still needed for output.

In one example of the disclosure, video decoder 300 may be configured todecode the first syntax element using a truncated binarization.

In another example of the disclosure, video decoder 300 may beconfigured to remove multiple pictures from the DPB. For example, videodecoder 300 may be configured to decode a second syntax elementindicating a number of pictures to remove from the DPB, and decode arespective first syntax element for each of the number of pictures, therespective first syntax elements indicating a respective picture toremove from the DPB. Video decoder 300 may then remove the respectivepictures indicated by the respective first syntax elements from the DPB.

In another example of the disclosure, when removing pictures from a DPBbased on an explicitly signaled syntax element (e.g., a DPB index),video decoder 300 may be configured to not perform a picture markingprocess.

In another example of the disclosure, video decoder 300 may beconfigured to conditionally decode inter prediction related syntaxelements based on a reference picture list size being greater than zero.

In another example of the disclosure, video decoder 300 may beconfigured to decode one or more pictures, wherein the one or morepictures includes the first picture, and store the one or more decodedpictures in the DPB. Video decoder 300 may be further configured to forma reference picture list using at least a subset of the one or moredecoded pictures stored in the DPB, and decode the current using aninter-prediction process and the reference picture list.

Other illustrative examples of the disclosure are described below.

Example 1—A method of coding video data, the method comprising: coding asyntax element indicating one or more pictures to remove from a decodedpicture buffer; and removing the one or more pictures from the decodedpicture buffer in accordance with the syntax element.

Example 2—The method of Example 1, wherein the syntax element is adpb_idx.

Example 3—The method of Example 2, wherein coding the syntax elementcomprises: coding dpb_idx using a truncated binarization.

Example 4—The method of Example 1, wherein the syntax element is adpb_idx_plus 1, where in the case that a value for dpb_idx_plus1 isgreater than 0, dpb_idx_plus1 minus 1 specifies an index to a picture inthe decoded picture buffer that is to be removed, and dpb_idx_plus1equal to 0 specifies that no picture in the decoded picture buffer is tobe removed.

Example 5—The method of Example 1, wherein coding the syntax elementindicating the one or more pictures to remove from the decoded picturebuffer comprises: coding the syntax element indicating multiple picturesto remove from the decoded picture buffer.

Example 6—The method of Example 1, further comprising: not performing apicture marking process.

Example 7—The method of Example 1, further comprising: conditionally notcoding inter prediction related syntax elements in the case that thereference picture list size is 0.

Example 8—The method of any of Examples 1-7, wherein coding comprisesdecoding.

Example 9—The method of any of Examples 1-7, wherein coding comprisesencoding.

Example 10—A device for coding video data, the device comprising one ormore means for performing the method of any of Examples 1-9.

Example 11—The device of Example 10, wherein the one or more meanscomprise one or more processors implemented in circuitry.

Example 12—The device of any of Examples 10 and 11, further comprising amemory to store the video data.

Example 13—The device of any of Examples 10-12, further comprising adisplay configured to display decoded video data.

Example 14—The device of any of Examples 10-13, wherein the devicecomprises one or more of a camera, a computer, a mobile device, abroadcast receiver device, or a set-top box.

Example 15—The device of any of Examples 10-14, wherein the devicecomprises a video decoder.

Example 16—The device of any of Examples 10-15, wherein the devicecomprises a video encoder.

Example 17—A computer-readable storage medium having stored thereoninstructions that, when executed, cause one or more processors toperform the method of any of Examples 1-7.

Example 18—A device for encoding video data, the device comprising:means for coding a syntax element indicating one or more pictures toremove from a decoded picture buffer; and means for removing the one ormore pictures from the decoded picture buffer in accordance with thesyntax element.

Example 19—Any combination of techniques described in this disclosure.

It is to be recognized that depending on the example, certain acts orevents of any of the techniques described herein can be performed in adifferent sequence, may be added, merged, or left out altogether (e.g.,not all described acts or events are necessary for the practice of thetechniques). Moreover, in certain examples, acts or events may beperformed concurrently, e.g., through multi-threaded processing,interrupt processing, or multiple processors, rather than sequentially.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over as oneor more instructions or code on a computer-readable medium and executedby a hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the techniques described inthis disclosure. A computer program product may include acomputer-readable medium.

By way of example, and not limitation, such computer-readable storagemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage, or other magnetic storage devices, flashmemory, or any other medium that can be used to store desired programcode in the form of instructions or data structures and that can beaccessed by a computer. Also, any connection is properly termed acomputer-readable medium. For example, if instructions are transmittedfrom a website, server, or other remote source using a coaxial cable,fiber optic cable, twisted pair, digital subscriber line (DSL), orwireless technologies such as infrared, radio, and microwave, then thecoaxial cable, fiber optic cable, twisted pair, DSL, or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. It should be understood, however, thatcomputer-readable storage media and data storage media do not includeconnections, carrier waves, signals, or other transitory media, but areinstead directed to non-transitory, tangible storage media. Disk anddisc, as used herein, includes compact disc (CD), laser disc, opticaldisc, digital versatile disc (DVD), floppy disk and Blu-ray disc, wheredisks usually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablegate arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the terms “processor” and “processingcircuitry,” as used herein may refer to any of the foregoing structuresor any other structure suitable for implementation of the techniquesdescribed herein. In addition, in some aspects, the functionalitydescribed herein may be provided within dedicated hardware and/orsoftware modules configured for encoding and decoding, or incorporatedin a combined codec. Also, the techniques could be fully implemented inone or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide varietyof devices or apparatuses, including a wireless handset, an integratedcircuit (IC) or a set of ICs (e.g., a chip set). Various components,modules, or units are described in this disclosure to emphasizefunctional aspects of devices configured to perform the disclosedtechniques, but do not necessarily require realization by differenthardware units. Rather, as described above, various units may becombined in a codec hardware unit or provided by a collection ofinteroperative hardware units, including one or more processors asdescribed above, in conjunction with suitable software and/or firmware.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A method of decoding video data, the methodcomprising: decoding a first syntax element indicating a first pictureto remove from a decoded picture buffer (DPB); removing the firstpicture from the DPB; decoding a current picture; and storing thedecoded current picture in the DPB.
 2. The method of claim 1, whereinthe first syntax element is a DPB index (dpb_idx), and wherein eachvalue of the DPB index is associated with a particular picture in thedecoded picture buffer.
 3. The method of claim 1, wherein the firstsyntax element is a DPB index plus 1 (dpb_idx_plus1), wherein, based ona value for dpb_idx_plus1 being greater than 0, dpb_idx_plus1 minus 1specifies an index to a particular picture in the decoded picture bufferthat is to be removed, and wherein, based on the value of dpb_idx_plus1being equal to 0, no picture in the decoded picture buffer is to beremoved.
 4. The method of claim 1, wherein the first syntax elementindicates the first picture to remove from the DPB using a value relatedto a picture order count (POC) value of the first picture.
 5. The methodof claim 1, wherein the first picture is not used for reference and isnot needed for output.
 6. The method of claim 1, wherein decoding thefirst syntax element comprises: decoding the first syntax element usinga truncated binarization.
 7. The method of claim 1, further comprising:decoding a second syntax element indicating a number of pictures toremove from the DPB; decoding a respective first syntax element for eachof the number of pictures, the respective first syntax elementsindicating a respective picture to remove from the DPB; and removing therespective pictures indicated by the respective first syntax elementsfrom the DPB.
 8. The method of claim 1, further comprising: notperforming a picture marking process.
 9. The method of claim 1, furthercomprising: conditionally decoding inter prediction related syntaxelements based on a reference picture list size being greater than zero.10. The method of claim 1, further comprising: decoding one or morepictures, wherein the one or more pictures includes the first picture;and storing the one or more decoded pictures in the DPB.
 11. The methodof claim 10, further comprising: forming a reference picture list usingat least a subset of the one or more decoded pictures stored in the DPB;and decoding the current using an inter-prediction process and thereference picture list.
 12. The method of claim 1, further comprising:displaying the current decoded picture.
 13. A device configured todecode video data, the device comprising: a memory configured to storevideo data; and one or more processors implemented in circuitry and incommunication with the memory, the one or more processors configured to:decode a first syntax element indicating a first picture to remove froma decoded picture buffer (DPB); remove the first picture from the DPB;decode a current picture; and store the decoded current picture in theDPB.
 14. The device of claim 13, wherein the first syntax element is aDPB index (dpb_idx), and wherein each value of the DPB index isassociated with a particular picture in the decoded picture buffer. 15.The device of claim 13, wherein the first syntax element is a DPB indexplus 1 (dpb_idx_plus1), wherein, based on a value for dpb_idx_plus1being greater than 0, dpb_idx_plus1 minus 1 specifies an index to aparticular picture in the decoded picture buffer that is to be removed,and wherein, based on the value of dpb_idx_plus1 being equal to 0, nopicture in the decoded picture buffer is to be removed.
 16. The deviceof claim 13, wherein the first syntax element indicates the firstpicture to remove from the DPB using a value related to a picture ordercount (POC) value of the first picture.
 17. The device of claim 13,wherein the first picture is not used for reference and is not neededfor output.
 18. The device of claim 13, wherein to decode the firstsyntax element, the one or more processors are further configured to:decode the first syntax element using a truncated binarization.
 19. Thedevice of claim 13, wherein the one or more processors are furtherconfigured to: decode a second syntax element indicating a number ofpictures to remove from the DPB; decode a respective first syntaxelement for each of the number of pictures, the respective first syntaxelements indicating a respective picture to remove from the DPB; andremove the respective pictures indicated by the respective first syntaxelements from the DPB.
 20. The device of claim 13, wherein the one ormore processors are further configured to: not perform a picture markingprocess.
 21. The device of claim 13, wherein the one or more processorsare further configured to: conditionally decode inter prediction relatedsyntax elements based on a reference picture list size being greaterthan zero.
 22. The device of claim 13, wherein the one or moreprocessors are further configured to: decode one or more pictures,wherein the one or more pictures includes the first picture; and storethe one or more decoded pictures in the DPB.
 23. The device of claim 22,wherein the one or more processors are further configured to: form areference picture list using at least a subset of the one or moredecoded pictures stored in the DPB; and decode the current using aninter-prediction process and the reference picture list.
 24. The deviceof claim 13, further comprising: a display configured to display thecurrent decoded picture.
 25. A device configured to decode video data,the device comprising: means for decoding a first syntax elementindicating a first picture to remove from a decoded picture buffer(DPB); means for removing the first picture from the DPB; means fordecoding a current picture; and means for storing the decoded currentpicture in the DPB.
 26. The device of claim 25, wherein the first syntaxelement is a DPB index (dpb_idx), and wherein each value of the DPBindex is associated with a particular picture in the decoded picturebuffer.
 27. The device of claim 25, wherein the first syntax element isa DPB index plus 1 (dpb_idx_plus1), wherein, based on a value fordpb_idx_plus1 being greater than 0, dpb_idx_plus1 minus 1 specifies anindex to a particular picture in the decoded picture buffer that is tobe removed, and wherein, based on the value of dpb_idx_plus1 being equalto 0, no picture in the decoded picture buffer is to be removed.
 28. Anon-transitory computer-readable medium storing instructions that, whenexecuted, causes one or more processors configured to decode video datato: decode a first syntax element indicating a first picture to removefrom a decoded picture buffer (DPB); remove the first picture from theDPB; decode a current picture; and store the decoded current picture inthe DPB.
 29. The non-transitory computer-readable medium of claim 28,wherein the first syntax element is a DPB index (dpb_idx), and whereineach value of the DPB index is associated with a particular picture inthe decoded picture buffer.
 30. The non-transitory computer-readablemedium of claim 28, wherein the first syntax element is a DPB index plus1 (dpb_idx_plus1), wherein, based on a value for dpb_idx_plus1 beinggreater than 0, dpb_idx_plus1 minus 1 specifies an index to a particularpicture in the decoded picture buffer that is to be removed, andwherein, based on the value of dpb_idx_plus1 being equal to 0, nopicture in the decoded picture buffer is to be removed.